1. Field of the Invention
The present invention relates to an assembly of protection diodes of low voltage level having a low protection voltage threshold that protect against electrostatic discharges. The present invention applies in particular to the protection of a hard disk read head.
2. Discussion of the Related Art
The increase of the storage capacities of hard disks requires the use of magneto-resistive read heads of very small dimensions. The new read heads allow disks to reach very high storage densities.
As very schematically illustrated in FIG. 1, a magneto-resistive head (MRH) 1 is generally connected to a support 2 by a thin sheet of a flexible material 3 on which are applied connection wires 4, 5, of the head. Flexible sheet 3 is of very small dimension. It has, for example, a length on the order of 5 to 10 mm and a width on the order of a few millimeters.
These heads are meant to be biased to a d.c. voltage on the order of a few hundreds of millivolts to optimize the reception of signals of very low amplitude on the order of one millivolt at frequencies higher than 100 MHz.
Given these very low voltage values, the heads, upon manufacturing and before their assembly in a circuit, are very sensitive to electrostatic discharges. It is known that electrostatic discharges can occur upon handling of the heads by tools or by hand. Electrostatic discharges can reach a few tens to a few hundreds of volts and are generally destructive to the headsso the manufacturing and assembly efficiency are extremely low.
These heads must be provided with protection against electrostatic discharges (ESD). The protection must:
be bi-directional, since ESDs can be positive or negative; PA1 have a very low protection voltage threshold; PA1 have a very low leakage current for the d.c. biasing of the head; and PA1 having a very low capacitance, so as not to attenuate the signals which, as previously indicated, can have a frequency higher than 100 MHz.
Further, the device has to be of very small dimension since it is desirable to be able to assemble it on flexible plate 3 without substantially increasing its inertia.
The conventional solution of protection against ESDs at very low threshold voltages is illustrated in FIG. 2. This solution includes arranging two pairs of diodes (D1-D2; D3-D4) in series, each of these pairs being in parallel and in opposition with respect to the other, across device 1 to be protected. However, the conventional solution to implement this assembly of four diodes D1 to D4 is not compatible with several of the conditions required for the protection of magneto-resistive heads. Indeed, discrete diodes, each mounted in a package or mounted together in a common package, are conventionally used. This has the disadvantage that the package inevitably has a non-negligible dimension and weight.